1. Field of the Invention
This invention relates to improvements in an air-fuel ratio control system for an internal combustion engine, and more particularly to the air-fuel ratio control system arranged to accomplish an air-fuel ratio control in response to two kinds of air-fuel rations detected respectively upstream and downstream of a catalytic converter.
2. Description of the Prior Art
Most automotive vehicles are equipped with a three-way catalytic converter disposed in an exhaust system of an internal combustion engine for the purpose of purifying exhaust gas discharged from the engine. Hitherto, in such automotive vehicles, an air-fuel ratio control system has been used to feedback-control the air-fuel ratio of air-fuel mixture to be supplied to the engine at a stoichiometric value, thereby effectively maintaining a desired conversion efficiently of the three-way catalytic converter. The air-fuel ratio feedback control system includes an oxygen sensor (air-fuel ratio sensor) adapted to detect an oxygen concentration in exhaust gas, thereby obtaining an actual air-fuel ratio of the air-fuel mixture to be supplied to the engine. The oxygen sensor is usually located, for example, at a position where the branch runners of an exhaust manifold is gathered with each other in order to ensure a high response of the air-fuel ratio feedback control. In accordance with the oxygen concentration in exhaust gas detected by the oxygen sensor, the actual air-fuel ratio of the air-fuel mixture is detected as to whether it falls in lean or rich side relative to the stoichiometric air-fuel ratio, thereby feedback-controlling the amount of fuel to be supplied to the engine, thus diverging the air-fuel ratio into the stoichiometric air-fuel ratio.
However, the above oxygen sensor is located relatively close to the combustion chambers of the engine and therefore is exposed to high temperature exhaust gas. As a result, the oxygen sensor tends to change in its characteristics such as internal resistance, electromotive force, response time under its thermal deterioration or the like. Additionally, since exhaust gases from respective engine cylinders cannot be sufficiently mixed with each other, it is difficult to obtain an average air-fuel ratio for all of the engine cylinders. Consequently, the oxygen sensor does not accurately read the air-fuel ration, rendering unprecise air-fuel ratio control.
In view of the above, a variety of air-fuel ratio feedback control systems and methods using an additional oxygen sensor disposed downstream of the catalytic converter have been proposed to accomplish the air-fuel ratio feedback control under the action of two oxygen sensors as disclosed, for example, in Japanese Patent Provisional Publication No. 58-72647.
In the air-fuel ratio feedback control method of the Japanese Patent Provisional Publication, a correction processing for an amount of fuel to be supplied to the engine is accomplished with a retard time under an output inversion (inversion between the lean and rich sides of air-fuel ratio) of the oxygen sensor located upstream of the catalytic converter. Here, the retard time is changed in accordance with the output of the oxygen sensor located downstream of the catalytic converter, thereby correcting the characteristics of the air-fuel ratio feedback control depending upon the upstream side oxygen sensor toward the direction in which the air-fuel ratio detected by the downstream side oxygen sensor approaches the stoichiometric air-fuel ratio as a target air-fuel ratio.
Now, with the above conventional air-fuel ratio control method, there is a possibility of an air-fuel ratio feedback control point depending upon the upstream side oxygen sensor largely shifting from the target point under the characteristics change of the upstream side oxygen sensor, thereby prolonging the retard time. This prolonged retard time causes a control cycle depending upon the upstream side oxygen sensor to be disturbed, thereby resulting in an abrupt change in air-fuel ratio of the air-fuel mixture to be supplied to the engine, thus degrading the stability of engine operation.
Here, it is assumed that the engine operation stability is prevented from being degraded by limiting the retard time to a sufficiently short value with a predetermined fixed maximum value. However, there is a concern that an excessively short retard time will not improve the air-fuel ratio precision, even if the retard time has not affected the operational characteristics of the engine.